Data networks continue to evolve with ever-higher speeds and more extensive topologies. In order to improve performance of such networks and troubleshoot problems, it is well known to monitor performance of networks through various techniques.
Conventional monitoring systems often use traffic analysis points (also known as test access points or TAPs) implemented with fused biconical tapered (FBT) devices to divert a portion of a data signal's power and send it to a monitoring device, while allowing the majority of the signal's power to continue on to its destination. However, use of conventional TAPs results in significant insertion loss, which can introduce signal degeneration and possibly data errors if the margin on the TAP side is not sufficient—exactly the opposite result intended by introduction of performance monitoring equipment.
Thus, challenges of network performance monitoring include minimizing the insertion loss that results from the monitoring equipment. Further challenges come from the fact that data networks operate in a multimode manner, adding complications of mode coupling and modal dispersion. Modal dispersion limits the bandwidth-distance of the network since different modes travel at different speeds. This not only causes a power penalty but also mode partition noise. Mode coupling in multimode fiber does not typically affect optical transmission, but with certain devices such as FBT splitters, higher order modes get lost in the fusion region of the splitter and as a result the device will be mode dependent, based on launching conditions. More generally, such characteristics result in significant insertion loss.
Conventional TAPs use transceiver subsystems based on PIN diodes (i.e., diodes having an “intrinsic” semiconductor region between the p-type region and the n-type region). These diodes have been used for decades for photodetection, but conventionally require significant numbers of photons to generate current. Thus, in typical applications, conventional monitoring solutions include TAPs that may sometimes utilize between 30 and 50% of the total power of a data signal for monitoring, essentially cutting the usable power of the data signal in half. Since, at higher transmission speeds, other significant signal strength losses already result from various network components (e.g., inherent loss in the fiber itself, in interconnects, multiplexer/demultiplexer devices, repeaters), the losses resulting from conventional monitoring solutions can impose strict limits on network capabilities.
For single mode fiber systems, other types of splitters (e.g., thin film splitters) and detectors (e.g., avalanche diode detectors) have variously been used, but such systems are not known to be applicable to multimode network systems. What is needed is a simple and inexpensive manner to obtain monitoring signals from a multimode optical network with minimal impact on the data network itself.